Pyrano pyrans

The herbicidins continue to prove a synthetic challenge, and this year Gallagher s group have reported the synthesis of 142 and 143 possessing the furano-pyrano-pyran skeleton of the herbicidins. Key synthetic stq>s are fluoride-mediated alkoxide cyclization onto a ketone functionality after desulfurization (Scheme 36). ... 

Pyrano[3,2-g]benzoxazine, dihydrosynthesis, 3, 714 Pyranobenzoxazines synthesis, 6, 190 Pyranobenzoxazoles mass spectra, 3, 615 Pyrano[2,3-a]carbazoles synthesis, 4, 235 Pyrano[2,3- 6]carbazoles synthesis, 4, 235 Pyrano[3,2-a]carbazoles synthesis, 4, 235 Pyranochromones synthesis, 3, 821 Pyranochromones, dihydrosynthesis, 3, 81 817 Pyranocoumarins crystal data, 3, 623 mass spectra, 3, 610 Pyranodipyranones synthesis, 3, 794 Pyrano[3,2-6]indol-4-ones synthesis, 4, 302 Pyran-2-ol, dihydrodehydration, 3, 762 Pyran-2-ol, 3-methyltetrahydro-synthesis, 3, 775 Pyran-2-ol, tetrahydro-6-substituted synthesis, 3, 775 Pyran-4-ol, tetrahydro-IR spectra, 3, 594 Raman spectra, 3, 594 Pyranols, tetrahydro-bond lengths, 3, 621 synthesis, 3, 777... 

Pyrano[4,3-6]pyran-2,5-dione, 7-methyl-aromaticity, 3, 639 Pyranopyrandiones aromaticity, 3, 639 dipole moments, 3, 627 synthesis, 3, 795 tautomerism, 3, 644 Pyranopyranones molecular structure, 3, 624 Pyranopyrans synthesis, 3, 760, 784 Pyranopyrazoles synthesis, 5, 321, 330 Pyranopyrazolones synthesis, 5, 334 Pyranopyridine synthesis, 3, 834 Pyranoses... 

Pyrano[3,2 ]indolizines such as 120 can be prepared from the reaction of N-( 1 -benzotriazolylmethyl)indolines with 3,4-dihydro-2//-pyran under acidic or Lewis-acidic conditions (Equation 26) <2001T4933>, and a simpler analogue 122 is the end product of an annulation process starting from the enamine 121 and ethyl acrylate <1996H(43)1391> (Scheme 34). Pyrano[4,3-/]indolizinetriones, for example, 124, result from the hydrolysis of protected 2,3-dihydro-l//-indolizincdioncs 123 (Equation 27) <2000H(53)771>. 

Pyrano- and thiopyranopyridines and their aza analogues fused to a five-membered heterocycle 12.17.3.1,5(i) Pyran or thiopyran as the outer ring... 

In a similar manner, pyrano[4,3-A]quinolizines 88 can be synthesized starting with substituted pyran-2-ones 87 (Scheme 5) <1999S1884>. 

Both natural and non-natural compounds with a 2ff,5ff-pyrano[4,3-fc]pyran-5-one skeleton are of interest in medicinal chemistry. Several natural products, such as the pyripyropenes, incorporate this bicyclic ring system. The group of Beifuss has described an efficient microwave-promoted domino synthesis of the 2ff,5H-pyr-ano[4,3-fo]pyran-5-one skeleton by condensation of a,/3-unsaturated aldehydes with 4-hydroxy-6-methyl-2]-f-pyran-2-one (Scheme 6.244) [428]. It is assumed that in the presence of an amino acid catalyst a Knoevenagel condensation occurs first, which is then followed by a 6jr-electron electrocyclization to the pyran ring. While the conventional thermal protocol required a reaction time of up to 25 h (refluxing ethyl... 

Scheme 6.244 Domino Knoevenagel condensation/6jr-electron electrocyclization in the synthesis of 2H,5H-pyrano[4,3-b]pyran-5-ones.

The synthesis of 4//-pyrano[2,3-d]pyrimidines, a class of compounds important in crop protection, is presented in Scheme 91. Dichlorotriphenyl-phosphorane affords iminophosphorane 247 with ethyl 2-amino-4//-pyran-3-carboxylate (246). Phenyl isocyanate cyclizes under alkoxide migration to afford pyrano[2,3-d]pyrimidines (248) (90LA995). 

N-Methyl-4-hydroxy-2-quinolone 183 in a three-component reaction gives spiro pyrano[3,2-c]qionoline-2-ones (02MI2), but indolones 213 with 4-hydroxy-6-methyl-2-pyridone 177 react in a complicated way (97BCJ1625). Ethoxycarbonyl-methylene indolone (Z = COOEt) forms pyrans 224, while dicyano analogs (Z = CN) yield substituted quinoline 225 (Scheme 85). 

Attempts to obtain pyrano[3,2-d]pyrimidine 272 from guanidine and the corresponding pyrans failed. The products were diaminopyrimidines 273, products of recyclization (86JPR35) (Scheme 110). 

The reaction was studied on nonannulated aminopyrans (89M1101), tetrahydrochromenes, naphtha[2,l-b]pyrans (04MI2), pyrano[3,2-/j]quino-lines (91BCJ668), pyrano[3,2-c]coumarines (01MOL519, 03MOL275), and... 

Addition of isothiocyanates to 2-amino-4H-pyran-3-carbonitriles with subsequent cyclization affords pyrano[2,3-d]pyrimidinethiones-2 290 (89JPR971, 08PS1145) (Scheme 121). 

In 1991, Furukawa et al. reported the isolation of pyrayafoline B (170) from the stem bark of M. euchrestifolia collected in May in Taiwan (87). In the same year, the same group isolated a further pyrano[2,3- ]carbazole alkaloid, pyrayafoline E (171), from the stem bark of M. euchrestifolia. Pyrayafoline E was isolated from Nature in racemic form (70). These alkaloids are the first members of the naturally occurring 2,7-dioxygenated-3-methylcarbazole alkaloids having a dimethyl pyran ring fused to C-6 and C-7 of the carbazole nucleus (70,87) (Scheme 2.35). 

Kamal A, Prasad BR, Ramana AV, Babu AH, Reddy KS (2004) FeCl3-NaI mediated reactions of aryl azides with 3,4-dihydro-2 //-pyran a convenient synthesis of pyrano-quinolines. Tetrahedron Lett 45 3507-3509... 

As part of extensive studies lasting over 30 years on the structures of chromophores involved in nonenzymatic browning reactions, two intensely orange, previously unknown, compounds have been identified (2R,8aR)-l and ZS, 8aR)-4-(2-furyl)-7-[(2-furyl)methylidene]-2-hydroxy-2//,7//,8a//-pyrano[2,3-3]pyran-3-one <1998CAR215>. Additional studies on the single Maillard reaction products of these compounds have also been reported <1998JFA3912>. 

Furopyranopyrandiones have been prepared by the cerium(iv) ammonium nitrate-mediated reaction of 4-hydroxy-277,577-pyrano[4,3- ]pyran-2,5-diones with alkenes (Scheme 14) <2003H(60)939>. 

Diels-Alder reactions have featured heavily during the period of review. Ar-Vinyl-2-oxazolidinone has been reported as a dienophile for the first time, including the preparation of various tetrahydro-277,77/-pyrano[4,3-. ]pyrans <2002SL952>. The heterodiene cycloaddition reaction of 3-formylchromone with a series of ketene acetals formed from C2-symmetric l,2-diarylethane-l,2-diols is completely diastereoselective (Scheme 40) <1995J(P1)2293>. 

Hanessian and Compain have also reported a Lewis acid-promoted inverse electron demand hetero-Diels-Alder reaction between dihydrofurans and dihydropyrans with a-keto-/3,7-unsaturated phosphonates to give stmcturally related products <2002T6521>. High-pressure OTr/o-selective hetero-Diels-Alder reactions between a,/3-unsaturated aldehydes and enol ethers in the presence of lanthanide catalysts have also been reported and give 3,4-dihydro-27/-pyrans. Examples include the use of cyclic enol ethers to give 2,3,4,4a,5,8a-hexahydro-277,577-pyrano[2,3-. ]pyrans <1995T8383>. 

Pyrano[3,4- / pyridazine derivatives 733-735 were prepared by cyclocondensation reactions of 6-aryl-3-(ethoxy-carbonyl, acetyl, or cyano)-2-oxo-2/7,3/7,4/7-pyran-4-carboxylic acids 730-732, respectively, with hydrazine hydrate in boiling ethanol (Equation 61) <2002PS1359>. 

Pyrano[2,3- ]-l,2,4-trioxines 260a-c arise in low yield from the reaction of the appropriate alkene, 3,4-dihydro-277-pyran 258, with singlet oxygen in the presence of methylene blue (MB), followed by condensation of the intermediate oxetane 259 with acetaldehyde or acetone as appropriate (Scheme 43) <1997H(46)451>. 

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